Problem: Simplify the following expression: $n = \dfrac{p}{4} - \dfrac{4p}{9}$
Answer: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $4$ and $9$ $\lcm(4, 9) = 36$ $ n = \dfrac{9}{9} \cdot \dfrac{p}{4} - \dfrac{4}{4} \cdot \dfrac{4p}{9} $ $n = \dfrac{9p}{36} - \dfrac{16p}{36}$ $n = \dfrac{9p -16p}{36}$ $n = \dfrac{-7p}{36}$